This invention relates to a tracking servo control apparatus for a disc drive system, and more particularly to a tracking control apparatus for a disc drive system in which a tracking servo signal is provided on a disc and fine tracking servo control is accomplished in accordance with the reproduced tracking servo signal.
Recently recording density, and especially track density, measured in units of TPI (tracks per inch), has become a very important factor in the field of the disc memory systems. To obtain higher track densities, more precise tracking systems are required. One solution to this tracking system problem has been the use of a tracking servo signal which is prerecorded on a disc. During operation, and in accordance with the reproduced servo signal, a closed loop tracking servo circuit controls the position of a head. This tracking servo technique is called a fine tracking servo control, while a coarse tracking servo accomplishes only a rough positioning of the head with the mechanical precision of an actuator. The coarse tracking servo control uses an open loop servo circuit. Generally, a combination of the fine and coarse tracking servo controls has been used for tracking control of the head in disc memory systems if they need a precise tracking control.
However, the above explained tracking control has several problems, especially when floppy discs are used as a disc medium. Since the floppy discs are interchangeable on a disc drive, some eccentricity of tracks cannot be avoided. The amount of the eccentricity changes in accordance with the condition of disc chucking onto the disc drive. Some expansion, contraction and deformation of the disc because of a change of temperature or humidity are also inherent characteristics of the floppy discs.
Accordingly, the positions of actual tracks on the disc are different from the positions of ideal tracks. Thus, it is very hard to make the head follow the actual tracks by using tracking error data when the prerecorded tracking signal is reproduced from the disc, because the amount and the direction of the track deviation from the ideal tracks is very large.